Idle Air Control (IAC) System Diagnosis

CIRCUIT DESCRIPTION
The powertrain control module (PCM) controls the idle air control (IAC) valve in order to regulate the air flow through the idle air bypass passage. The amount of air flowing through the idle air bypass passage determines the idle speed. The PCM directs the IAC valve according to changes in engine load. A stalling or poor idle concern could be caused by an IAC valve that is not operating properly.

DIAGNOSTIC AIDS
Any condition that can affect engine performance at idle, can affect the performance of the IAC system. A careful visual and/or physical inspection of the engine is necessary when the scan tool indicates that the engine speed or the IAC Motor Command readings are out of range. An intermittent IAC system concern may be caused by poor electrical connections in the IAC valve circuits. Thoroughly check any circuitry that is suspected of causing an intermittent condition. Refer to Intermittent Conditions. Intermittent Conditions

TEST DESCRIPTION

Steps 1-2:

Steps 3-4:

Steps 5-7:

Steps 8-10:

Steps 11-15:

Steps 16-18:

The numbers below refer to the step numbers on the diagnostic table.
1. A Diagnostic System Check-Computers and Controls Systems prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
4. This step tests whether the PCM can operate the IAC valve with the scan tool. The IAC valve can be commanded to increase and/or decrease engine speed by using the scan tool RPM Control function. Remember to clear any DTCs and run the engine until the engine reaches a normal operating temperature before attempting RPM Control with the scan tool.
5. This step checks the operation of both electrical load idle-up circuits. If there is no increase in the IAC Motor Command percentage when the electrical accessory is turned ON, the idle-up circuit may be inoperative or always ON.
6. This step checks the operation of the A/C idle-up circuit. If there is no increase in the IAC Motor Command percentage when the A/C compressor is engaged, the idle-up circuit may be inoperative or always ON.
10. This step simulates the pulse width modulated (PWM) signal supplied to the IAC valve by the PCM by rapidly grounding and un-grounding terminal 1 of the IAC valve.
12. The most likely cause of no ignition positive voltage is an open circuit condition between the IAC valve connector terminal 2 and the splice. A shorted or inoperative EFI relay circuit will cause a no-start condition.